1. Field of the Invention
This invention relates to a method for the removal of solid particulates and acidic gases from gaseous mixtures. More particularly, this invention relates to a process for reducing flue gases contaminants which are ordinarily present in flue gases produced in the process of regenerating catalysts employed in the catalytic cracking of hydrocarbons.
2. Description of the Prior Art
The flue gases produced in the process of regenerating catalysts employed in the catalytic cracking of hydrocarbons contain solid particulates, including catalyst fines, and various noxious gases, such as carbon monoxide, sulfur oxides, and ammonia. It is desirable to lower the level of such contaminants from the flue gas before it is introduced into the atmosphere so as to minimize the detrimental effect of these pollutants to the ecology.
Many methods have been used heretofore to reduce the solid particulate content, the acidic gas content or both of a flue gas stream. These conventional methods include centrifugal particle separation, electrostatic precipitation, adsorption, catalytic oxidation, and a variety of other processes. The greatest disadvantage encountered in all such processes is the expense required to effect decontamination. The reason for the expense usually stems from the initial large expense of the equipment involved, such as is necessary for an electrostatic precipitator. Other processes can be operated only at a rather large recurring cost, such as adsorption processes in which the adsorbent cannot be regenerated and must continually be replaced.
It is also known that solid particulates can be removed from gases by wet scrubbing methods, including scrubbing gases and jet-ejector type venturi scrubbers in which a scrubbing liquid, under pressure, enters the venturi through a spray nozzle. The velocity of the liquid spray creates a draft in a chamber of the venturi scrubber and causes gases or vapors to be drawn into the scrubber body and through a constricted passage of the scrubber wherein intimate mixing of scrubbing liquid and gas occurs. Generally, the effluent of the scrubber is passed through a separator in which the contaminated liquid is separated from the clean gas. It is also known that acidic or basic materials may be added to the scrubbing liquid to neutralize or adsorb basic or acidic contaminants that may be present in the gas subjected to the wet scrubbing treatment.
Although many types of venturi web gas scrubbing systems have been proposed to reduce the level of particulates and acidic gases from gaseous mixtures from various processes prior to their discharge into the atmosphere, these systems have not found widespread use for reducing the contaminants normally present in flue gases produced in the process of regenerating catalysts employed in the catalytic cracking of hydrocarbons. This is, in part, due to the particular nature of the contaminants present in the flue gas, various pressure requirements and the large volumes of gases which must be handled before the flue gas from the catalyst regenerator can be discharged into the atmosphere. For example, the flue gas from a cracking catalyst regenerator will ordinarily be at a pressure of 10-30 psig and contain as much as 6-13 vol. % carbon monoxide. In order to meet pollution regulations, it is very often necessary to reduce the carbon monoxide content of the flue gas by burning the carbon monoxide in a CO boiler. Since an efficient CO boiler requires a low pressure level in the order of 0.2-1 psig, it is necessary to reduce the pressure of the flue gas from the catalyst regenerator by passage through a pressure reduction zone, such as an orifice chamber, prior to entry into the CO boiler. Unfortunately, the large volume of gas from the CO boiler is thus not at a sufficiently high enough pressure to permit the use of conventional venturi wet scrubbing systems. Accordingly, the use of a jet-ejector type of venturi scrubber has recently been proposed in application Ser. No. 369,771, filed June 13, 1973, now U.S. Pat. No. 3,970,740.
The jet-ejector venturi combines the clean-up characteristics of a venturi scrubber with the gas transport characteristics of an ejector. The caustic scrubbing liquid is supplied to the scrubber under pressure ranging from 40-120 psig in amounts varying between 20 and 100 gals./1000 cu. ft. of entering gas. The scrubbing liquid then passes through a specifically designed spray nozzle which breaks the liquid stream into droplets. These droplets have size and velocity characteristics most suitable for maximum contacting with the gas to provide the best possible scrubber efficiency. It is the velocity of the spray liquid which induces a draft into the scrubber body. The dust-laden flue gases are drawn into the body of the scrubber by the draft-inducing action of the liquid spray. The gas is intermixed with a scrubbing liquid and both enter the venturi portion of the scrubber axially. In the venturi section of the scrubber, the liquid and gas enter an area of intense turbulence. Hence, the liquid droplets strike and capture the particles in the gas stream. Also within this venturi section, compression of the gas occurs creating a differential pressure across the unit. After passing through the venturi section, the mixture of gas and droplets is sent to a separator. In the separator, the clean gas is separated from the dirty water and discharged to the atmosphere.
While many advantages are realized by employing the jet-ejector venturi scrubber for reducing the contaminants ordinarily present in flue gases produced in the process of regenerating catalysts employed in the catalytic cracking of hydrocarbons, the system has the disadvantage of requiring large volumes of high pressure water and the need for relatively expensive equipment. It has now been found that many of the disadvantages associated with the prior art devices are overcome by the practice of the present invention which produces flue gases having a low level of contamination without the need of CO boilers, high pressure/high volume water pumps, etc. which are normally associated with prior art processes.